Small form factor hard disk drives are typically utilized in portable appliances such as, for example and without limitation, digital cameras that are typically subjected to shocks and vibration. As is well known, to enhance the durability of such hard disk drives, whenever the drive in not being used, the disk drive heads are typically parked on a ramp away from the disk surface.
Under a normal turn-off condition, before power to the disk drive is turned off, the disk drive undergoes a predetermined sequence of steps to safely park the disk drive heads on the ramp. However, in abnormal circumstances such as, for example and without limitation, when power to the disk drive is suddenly disconnected (sometimes this is referred to as an Emergency Power Off (“EPO”) event), the predetermined sequence of steps to park the heads on the ramp cannot take place. As is well known, in such abnormal circumstances, an EPO procedure is invoked wherein the disk drive heads are unloaded using energy stored in the disk drive. To prevent disk drive failure, the stored energy must be sufficient to enable the disk drive heads to be totally withdrawn from the disk surface, and parked on the ramp. In accordance with prior art methods utilized on large form factor hard disk drives, upon the occurrence of an EPO event, the disk drive will park the disk drive heads in a safe place (for example, on a safe place on the disk itself), using kinetic energy stored in a spindle and sometimes supplemented with energy stored in capacitors. If the supplemental storage capacitance is large, this may require significant amounts of space inside the disk drive.
In the case of a small form factor disk drive, energy storage issues related to the above-described method of dealing with an EPO event are exacerbated for several reasons. First, a small form factor disk drive typically uses a small spindle having negligible kinetic energy. As a result, the energy required to deal with an EPO event must all, or at least in substantial part, come from energy stored in capacitors. Second, as was set forth above, for durability, the disk drive heads must be unloaded onto a ramp, and this requires more energy that landing on a safe place such as, for example and without limitation, a safe place on the disk. Third, space on the disk drive is greatly restricted. As a result, the size of capacitors that can be used is limited.
The issue related to capacitor size can be understood as follows. For a given capacitor body size, a product of capacitance C and operating voltage V is a fixed value K, where K=C*V. Thus, for a fixed body size, doubling the operating voltage of the capacitor will reduce the capacitance value by a factor of 2. However, the energy E stored in a capacitor is proportional to a product of capacitance and the square of the operating voltage, i.e., E=k*C*V2 where k is a constant of proportionality. Hence, for a fixed body size, doubling the operating voltage of the capacitor, will increase the stored energy by a factor of 4 (due to the increase in operating voltage) and will reduce the stored energy by a factor of 2 (due to the smaller capacitance), resulting in a net doubling of the stored energy in the fixed body size. This has been used in the prior art to reduce the overall physical size of capacitors required to store the requisite amount of energy needed to deal with an EPO. In particular, such an elevated voltage source for this purpose is already available in a disk drive due to the need for such voltages to satisfy other disk drive functions. This elevated voltage may also be used to charge the storage capacitors utilized as a source of energy for dealing with an EPO event. However, to prevent damage to the storage capacitors, their safe operating voltages must exceed an absolute maximum expected voltage available from the source of elevated voltage described above.
As is known, nominal or minimum values of the elevated voltage will vary due to spread in circuit component characteristics, power supply variations, and other factors. These variations are problematic because they can reduce the elevated voltage by 20% or more below that of its maximum value. As a result, as was described above, this can result in a possibility of a 40% or more reduction in stored energy in the capacitors, and this may result in an inadequate amount of energy to deal with an EPO event.
In light of the above, there is a need to overcome one or more of the above-identified problems.